The present invention relates generally to the removal of particulate deposits or dust-cake from surfaces of filters used for cleaning particulate-laden gas streams at high temperature and high pressure, and more particularly to the apparatus and method for cleaning such filters by back-flushing the filters with a pulse of hot gas from a combustion mechanism.
The gasification or combustion of carbonaceous materials such as coal, peat, bio-mass, and waste products produce process gases which are laden with particulate material. Before these hot gases can be discharged into the atmosphere or utilized in energy consuming systems such as gas turbines all, or essentially all, of the particulates in the process gases have to be removed from the process gases in order to satisfy environmental regulations as well as to protect system components such as gas turbines or heat exchangers. Waste incinerators also produce process gas streams which require particulate removal before the gas streams are discharged in the environment or otherwise used. Process gas streams such as utilized as the motive fluid for a gas turbine are provided to the turbine at a relatively high temperature and pressure for effecting the efficient extraction of the energy values in the process gas.
Various filtering techniques have been utilized to remove particulate material from process gas streams, including those at elevated temperatures and pressures. For example, barrier filters, "screenless grannular" bed filters, acoustic agglomeration, nested fiber filters, and cyclones have been satisfactorily used for stripping particulates from high-temperature and high-pressure gas streams.
Of these techniques, it has been found that barrier filter systems employing ceramic filter elements provide one of the most effective mechanisms for removing the particulate material from process gas streams at high temperature and high pressure. Barrier filter systems are normally provided by supporting a plurality of ceramic gas filter elements within a pressure vessel. A tubesheet with the filter elements separates the pressure vessel into a dirty gas zone which contains the particulate laden gas and a clean gas zone which contains the essentially particulate-free gas. Normally, the gas filter elements are attached directly to the tubesheet but they can be mounted in plenums supported by the tubesheet. Barrier filter systems utilize filter elements of different configurations such as cross-flow filters as disclosed in U.S. Pat. Nos. 4,343,631, 4,735,638, 4,737,176, and 4,764,190, or candle filters as disclosed in U.S. Pat. No. 2,548,878 and German patents 3515345 and 3515364. The preferred barrier filter elements are formed of ceramic materials such as silicon carbide, mullite, or aluminates since such materials exhibit high resistance to corrosion and mechanical degradation by exposure to particulate-laden process gases at high temperature and high pressure such as produced by the combustion or gasification of coal.
In the operation of a barrier filter, a substantial portion of the particulates in the process gas in the dirty gas zone contacting the exposed surfaces of the gas permeable filter elements remain on or just below the surface of the filter elements to form a layer of particulate material, usually referred to as filter-cake or dust-cake. With pore construction of the filter element, differential pressure across the filter elements increases with increasing thickness of the dust-cake until the pressure drop across the filter elements reaches an unsatisfactorily high level so as to require the removal of the dust-cake. Usually, the removal of this dust-cake is periodically required to maintain acceptable pressure drops across the filter elements. The dust-cake build-up and cleaning times are determined by either selecting a maximum allowable pressure drop or by cleaning the filters at preset intervals.
The removal of the dust-cake from the filter elements in the barrier filter system can be effected by back-flushing the filter elements with high pressure gas. A previously employed back-flushing technique used a stream of relatively cold nitrogen or air at high pressure to clean the filter elements of either the candle or the cross-flow type. In using this back-flushing technique, a high pressure vessel is required for storing or a high-pressure compressor for supplying the gas used for the cleaning operation. Also, rapid responding valving systems are required to provide high pressure pulses of nitrogen or air to the "clean side" of the filter elements for back-flushing the latter. This cleaning operation can be conducted while the barrier filter is on-line, i.e., while the process gas is flowing through the filter system, or off-line, i.e., when the barrier filter is isolated from the process gas stream. It has been found that the on-line cleaning of the barrier filters substantially reduces the number of barrier filter vessels required so as to significantly reduce the overall cost of the filter system.
However, the use of the relatively cold back-flushing gases has caused some problems since these gases are injected into the barrier filters at temperatures considerably less than that of on-line filter components so as to thermally shock and often cause a premature failure of the ceramic filter elements, produced by contacting the filter elements with the pulse of cold air or nitrogen. Efforts have been made to reduce thermal shock problems by warming the relatively cold back-flushing gases prior to any contact thereof with the filter elements. One such effort incorporated a venturi above each of the filter elements so that the pulse gas functions as an inductor for pulling hot process gas into the filter element during the back-flushing operation. While this technique did provide some warming of the air, it was found that when the filter system is on-line the presence of these venturies resulted in an undesirable filter system pressure loss during operation, excluding the cleaning periods.
Another technique utilized for cleaning filter elements was the use of acoustic energy from a pulse combustor as practiced in the cleaning of nested fibers and described in the publication "Advanced Development of the Nested Fiber Filter," Battelle Status Report, Battelle National Laboratory at Columbus, Ohio, May, 1990. There will be difficulties with the pulse technique in a full-scale barrier filter system in which several thousand filter elements are normally utilized. The problems associated with monitoring or controlling the operation of several thousand pulse combustors or even manifolding these several thousand pulse combustors is considered to be impractical, if not impossible.
A pulse combustor operates on the basis of acoustic coupling with the chamber, and would be upwards of twenty cylces per second, whereas a pulse is generated after hundreds of minutes of operation and for periods of seconds, hence long time scales compared to pulse combustion. While a pulse combustor could be made to operate over the short burst-time, the shaping of the pulse is not practical and proper timing is expected to be impossible since the pulse combustor takes a "long" time to achieve thermal conditions that produce acoustic coupling.
Another problem foreseen in using acoustic energy from pulse-type combustors for filter cleaning is that the filter elements could be excessively and deleteriously vibrated by the acoustic energy in the filter cleaning operation.